1. Field of the Invention
This invention relates to measuring instruments utilizing a variable color digital display.
2. Description of the Prior Art
A display device described in U.S. Pat. No. 3,740,570, issued on Jun. 19, 1973 to George R. Kaelin et al., uses special LEDs that exhibit different colors when subjected to different currents. The LEDs are biased by pulses of different amplitudes, to achieve different colors of the display, and by the length inversely proportional to the pulse amplitude, to compensate for variable brightness of the display caused by different amplitudes of the pulses.
A circuit employing a dual-color LED driven by a dual timer is described in the article by Bill Wagner entitled 2-color LED+driver=versatile visual effects, published on Oct. 2, 1980 in EDN volume 25, No. 19, page 164. Since dual-color LEDs are connected to conduct currents in opposite directions, it would be impossible to forwardly bias them simultaneously.
An electronic display having segments wherein each segment is capable of selectively illuminating two colors is disclosed in U.S. Pat. No. 4,488,149 issued on Dec. 11, 1984 to William A. Givens, Jr. Two AND gates are provided for biasing, in each display segment, either a first light emitting diode, for emitting a first color, or a second light emitting diode, for emitting a second color. It is not contemplated to illuminate both light emitting diodes in each segment simultaneously for blending the colors.
A multi-colour LED display for seven segment figures is disclosed in German Patent No. 3,009,416 issued on Sep. 17, 1981 to Klaus Gillessen. The LED circuit consists of seven group of diodes, each group made up of two diodes, one red and one green. The cathodes of all red diodes are terminated in a common connection, and the green ones likewise, the anodes of each pair of diodes being common for that pair. Two transistors connected in cascade are utilized for activating either all green LEDs, or all red LEDs, but not both simultaneously. When a low level signal is applied to the base of the first transistor, the first transistor is blocked, and the second transistor is turned ON to illuminate the second set of the diodes. When a high level signal is applied to the input of the first transistor, the first transistor is turned ON, while the second transistor is blocked, whereby the first set of the diodes is illuminated. Since the first transistor cannot be simultaneously turned ON and OFF, it would be impossible to illuminate the two sets of the diodes simultaneously. In another embodiment is shown a multi-colour LED display utilizing three sets of LEDs: red, green, and blue, which are respectively commonly connected and may be activated by manual switches. It is not contemplated that the three sets of LEDs be activated in selective combinations to blend the colors.
A digital electrooptical display with anti-parallel light emitting diodes is disclosed in East German Patent No. 220,844 issued on Apr. 10, 1985 to Thomas Hoffmann et al. Two light emitting diodes for emitting light of different colors are connected in each segment back-to-back. Since the light emitting diodes are connected to conduct currents in opposite directions, it would be impossible to illuminate them simultaneously, because the opposite currents attempting to pass through a single conductor would cancel.
A system for measuring radioactivity is described in U.S. Pat. No. 2,643,344, issued on Jun. 23, 1953 to Ian H. McLaren et al. Another systems for measuring radioactivity are described in U.S. Pat. No. 2,648,015 issued on Aug. 4, 1953 to Alexander Greenfield et al., and in U.S. Pat. No. 2,682,000 issued on Jun. 22, 1954 to Joseph F. Clayton et al. Since these three patents are very similar, they are equally authoritative. The system for measuring radioactivity, disclosed in the three patents, includes an analog pointer indicator having three scales, with three associated colored light bulbs, for indicating which scale is applicable for the reading of the measured value. A mechanism for automatic range selection is provided, which is controlled by two solenoids, and which also controls the three associated colored light bulbs. In response to the range selection, one of the light bulbs illuminates the indicator face with a distinctive color to indicate which measurement scale is applicable.
A digital meter is disclosed in U.S. Pat. No. 2,889,518, issued on Jun. 2, 1959 to Harold R. Hudson et al. The digital meter includes a motor, a wheel counter rotatable by the motor, a potentiometer rotatable by the motor, and a mechanical sampler (vibrator). An unknown voltage is compared, by the sampler, with an instant reference voltage that appears on the wiper of the potentiometer. The motor keeps rotating the potentiometer until the unknown voltage is equal to the reference voltage. The rotation of the motor is then automatically stopped, which also stops the wheel counter, whose position numerically indicates the value of the measured voltage. The decimal point monochromatic lamps are placed between the wheels of the counter. One of the decimal point lamps is illuminated, by manually operated range switch, to indicate by its position the selected measurement range.
A high speed multi-channel voltage indicator is disclosed in U.S. Pat. No. 2,878,450 issued on Mar. 17, 1959 to Jean C. Rabier. The magnitudes of the input voltages are adjusted by a resistor network, subtracted from a single reference voltage source, and sequentially connected by a rotatable arm of a motor to an evaluator circuit, which produces on an oscilloscope screen one of three readings, "Too High", "Too Low", or "Go". The test results may be indicated on the oscilloscope screen either by a red color square, or by a green color square, or by a blue color square.
The prior art testing instruments do not contemplate an error memory for storing data indicative of the number of previous errors. The prior art testing instruments do not contemplate that a digital indication of the presently measured value may be colored in accordance with its relation to the number of past measurement errors.